1. Field of the Invention
The present invention relates to a method and apparatus of driving a plasma display panel, and more particularly to a method and apparatus of driving a plasma display panel that is adaptive for expressing linear gray levels by way of preventing a gray level inversion.
2. Description of the Related Art
A plasma display panel PDP is a display device using a phenomenon that visible ray is generated from a fluorescent substance when vacuum ultraviolet ray generated by gas discharge excites the fluorescent substance. The PDP is thinner and lighter than a cathode ray tube CRT, which has been used as main display means so far, and can be embodied of high definition and wide screen. The PDP includes a plurality of discharge cells arranged in a matrix, and one discharge cell constitutes one pixel of a screen.
FIG. 1 is a perspective view illustrating a discharge cell structure of a three-electrode AC surface discharge PDP of the related art.
Referring to FIG. 1, the discharge cell of the three-electrode AC surface discharge PDP of the related art includes a first electrode 12Y and a second electrode 12Z formed on an upper substrate 10, and an address electrode 20X formed on a lower substrate 18.
An upper dielectric layer 14 and a passivation film 16 are deposited on the upper substrate 10 provided with the first and second electrodes 12Y and 12Z. Wall charges generated upon plasma discharge are accumulated in the upper dielectric layer 14. The passivation film 16 prevents the damage of the upper dielectric layer 14 caused by the sputtering generated upon the plasma discharge and, at the same timer increases the emission efficiency of secondary electrons. The passivation film 16 is usually magnesium oxide MgO.
A lower dielectric layer 22 and barrier ribs 24 on the lower substrate 18 provided with the address electrode 20X, and a phosphorus layer 26 is spread on the surface of the lower dielectric layer 22 and the barrier ribs 24. The address electrode 20X is formed crossing the first and second electrode 12Y and 12Z.
The barrier ribs 24 are formed parallel to the address electrode 20X to prevent an ultraviolet ray and a visible ray from leaking out to adjacent discharge cells, wherein the ultraviolet ray and the visible ray are generated by discharge. The phosphorus layer 26 is excited by the ultraviolet ray generated upon the plasma discharge to generate any one of red, green or blue visible ray. An inert mixed gas for gas discharge is injected into a discharge space provided between the upper substrate 10, the lower substrate 18 and the barrier ribs 24.
The PDP is driven by way of dividing one frame into several sub-fields that have a different number of discharges, for realizing the gray level of a picture. Each sub-field can be divided again into a reset period to generate a uniform discharge, an address period to select discharge cells and a sustain period to realize gray levels in accordance with the number of discharges.
For instance, in the event that it is wanted to display a picture with 256 gray levels, a frame period, 16.67 ms, corresponding to 1/60 second is divided into 8 sub-fields, as in FIG. 2. In addition, each of 8 sub-fields SF1 To SF8 is divided again into the reset period, the address period and the sustain period, Herein, the reset period and the address period of each sub-field are the same for each sub-field, whereas the sustain period increases at the rate of 2n (n=0,1,2,3,4,5,6,7) in each sub-field.
FIG. 3 is a waveform diagram representing a driving method of a three-electrode AC surface discharge PDP of the related art.
Referring to FIG. 3, one sub-field is divided into a reset period to initialize the whole screen, an address period to write data while scanning the whole screen in the line sequential color TV system, and a sustain period to keep the light emission state of cells to which the data are written.
Firstly, in the reset period, reset waveforms RP are applied to first electrode lines Y1 to Ym. If the reset waveforms are applied to the first electrode lines Y1 to Ym, reset discharges are generated between the first electrode lines Y1 to Ym and second electrode lines Z1 to Zm to initialize discharge cells.
In the address period, scan pulses SP are sequentially applied to the first electrode lines Y1 to Ym. Address electrode lines X1 to Xn are supplied with data pulses Dp synchronized with the scan pulses SP. At this moment, address discharges are generated in the discharge cells to which the data pulses Dp and the scan pulses SP.
In the sustain period, first and second sustain pulses SUSPy and SUSPz are supplied to the first electrode lines Y1 to Ym and the second electrode lines Z1 to Zm. At this moment, sustain discharges are generated at the discharge cells where the address discharges are generated.
The brightness of such a PDP is determined as in Formula 1.
                              B          graylevel                =                  gain          ×                                    ∑                              i                =                1                            k                        ⁢                                          A                i                            ×                              N                i                            ×              s                                                          [                  FORMULA          ⁢                                          ⁢          1                ]            
Herein, ‘B’ represents brightness, ‘A’ represents sub-field map information, ‘k’ represents the number of sub-fields, ‘N’ represents sub-field weight, and ‘s’ represents one time discharge brightness of sustain pulse.
Gain can be obtained using the ratio of the number of sustains to the number of gray levels. In other words, gain=the total number of sustains/(gray level−1). For instance, if the total number of sustains is 255 and the gray level is 256, the gain can be set to be ‘1’.
The sub-field mapping information A represents selection information during the address period. For example, if a discharge cell is selected during the address period, the sub-field mapping information (A) is set to be ‘1’, and if a discharge cell is not selected during the address period, the sub-field mapping information A is set to be ‘0’. The sub-field weight (N) corresponds to the number of current sub-fields (k). ‘s’ represents the brightness generated by one time sustain discharge.
For example, in the plasma display panel, if the gain is to be ‘1’, twelve sub-fields are included and the sub-fields weights are set to be ‘1, 2, 4, 8, 16, 32, 32, 32, 32, 32, 32, 32’, the brightness of the PDP can be set as in Table 1.
TABLE 1GraySub-field weightBright-level12481632323232323232ness 0XXXXXXXXXXXX 0 S 10XXXXXXXXXXX 1 S 2X0XXXXXXXXXX 2 S. . . . . . . . .  3100000XXXXXXX 31 S 32XXXXX0XXXXXX 32 S. . . . . . . . . 255000000000000255 S
Herein, ‘X’ represents that gray level is not expressed, and ‘0’ represents that gray level is expressed. As can be seen in Table 1, the PDP includes twelve sub-fields, and gray levels of 256 is expressed in use of the brightness weight of ‘1, 2, 4, 8, 16, 32, 32, 32, 32, 32, 32, 32’.
Table 1 represents the brightness of the PDP in consideration of only the light generated by sustain discharge. However, the PDP, when actually driven, generates light not only by a sustain discharge but by a reset discharge and an address discharge. In this way, if gray level is expressed including the reset discharge, the address discharge and the sustain discharge, there occurs a gray level inversion as in FIG. 4. In other words, it happens that the brightness of the PDP expressed in the gray level of n−1 is brighter than the brightness of the PDP expressed in the gray level of n (n is a natural number).
To describe this more specifically, sub-fields with the brightness weights of 1, 2, 4, 8, 16 are to selected to express the gray level of 31, as shown in Table 1. Accordingly, the address discharges are generated in five sub-fields in order to express the gray level of 31. As compared with this, one sub-field with the brightness weight of 32 is to be selected in order to express the gray level of 32. Accordingly, the address discharge is generated in one sub-field in order to express the gray level of 32. At this moment, the light generated by the address discharge is caused to generate a brightness inversion between the gray levels of 31 and 32. In other words, the gray level of 31 generates brighter light than the gray level of 32.
The brightness of the PDP including the light generated in the actual reset discharge and address discharge is determined as in Formula 2.
                                          B            graylevel                    ⁡                      (                          r              ,              a              ,              s                        )                          =                              L            ×            r                    +                                    ∑                              i                =                1                            k                        ⁢                                          A                i                            ×              a                                +                      gain            ×                                          ∑                                  i                  =                  1                                k                            ⁢                                                A                  i                                ×                                  N                  i                                ×                s                                                                        [                  FORMULA          ⁢                                          ⁢          2                ]            
Herein, ‘L’ represents the number of sub-fields that are reset at the beginning, ‘r’ is one time discharge brightness of reset pulse, and ‘a’ is one time discharge brightness of address pulse.
‘L’ represents the number of sub-fields where the reset discharges are generated. For example, if the PDP includes twelve sub-fields and the reset discharges are generated in twelve sub-fields, ‘L’ is set to be ‘12’.
A matrix as in Formula 3 can be derived from Formula 2.
                                          (                                                            12                                                  0                                                  0                                                                              12                                                  1                                                  1                                                                              …                                                  …                                                  …                                                                              12                                                  12                                                  255                                                      )                    ⁢                      (                                                            r                                                                              a                                                                              s                                                      )                          =                  (                                                    0.9487                                                                    2.275625                                                                    …                                                                    124.85                                              )                                    [                  FORMULA          ⁢                                          ⁢          3                ]            
On the other hand, in the related art PDP, a sustain pulse pair is additionally supplied for each sub-field in order to stabilize the sustain discharge during the sustain period. The brightness of the PDP including the light generated from the sustain pulse pair is determined as in Formula 4.
                                                                                          B                  graylevel                                ⁡                                  (                                      r                    ,                    a                    ,                    s                                    )                                            =                            ⁢                                                L                  ×                  r                                +                                                      ∑                                          i                      =                      1                                        k                                    ⁢                                      Ai                    ×                    a                                                  +                                                                                                      ⁢                                                gain                  ×                                                            ∑                                              i                        =                        1                                            k                                        ⁢                                          Ai                      ×                                              N                        i                                            ×                      s                                                                      +                                                      ∑                                          i                      =                      1                                        k                                    ⁢                                      Ai                    ×                    s                                                                                                          [                  FORMULA          ⁢                                          ⁢          4                ]            
In this way, the matrix as in Formula 3 is derived from Formula 4, the values of ‘r’, ‘a’ and ‘s’ can be obtained in use of this. In fact, the value of ‘r’ (one time discharge brightness of the reset pulse) is 0.208815[cd/m2], the value of ‘a’ (one time discharge brightness of the address pulse) is 0.413396[cd/m2], and the value of ‘s’ (one time discharge brightness of the sustain pulse) is 0.44553[cd/m2]. Herein, the values of ‘r’, ‘a’ and ‘s’ are not actual brightness but calculated values from formulas. If the values of ‘r’, ‘a’ and ‘s’ are substituted into each formula, it is possible to obtain a brightness similar to the actual brightness.
In this way, the brightness of the PDP including the discharge brightness of the reset pulse, the discharge brightness of the address pulse and the discharge brightness of the sustain pulses, i.e., the brightness of the PDP by Formula 4, can be shown as in Table 2.
TABLE 2GraySub-field weightlevel12481632323232323232Brightness 0XXXXXXXXXXXX12 r + 0 a + 0 s + 0 s 10XXXXXXXXXXX12 r + 1 a + 1 s + 1 s 2X0XXXXXXXXXX12 r + 1 a + 2 s + 1 s. . . . . . . . .  3100000XXXXXXX12 r + 5 a + 31 s + 5 s 32XXXXX0XXXXXX12 r + 1 a + 32 s + 1 s. . . . . . . . . 25500000000000012 r + 12 a + 255 s + 12 s
In Table 2, only the brightness of reset pulse generated at twelve sub-fields in the gray level of ‘0’. In the gray level of ‘1’, there are shown the sustain brightness corresponding to the brightness weight of ‘1’, the brightness by one sustain pulse pair, the brightness by twelve reset pulses, and the brightness by one address discharge. Further, in the gray level of ‘31’, there are shown the sustain brightness corresponding to the brightness weight of ‘31’, the brightness by five sustain pulse pairs, the brightness by twelve reset pulses, and the brightness by five address discharges. And, in the gray level of ‘32’, there are shown the sustain brightness corresponding to the brightness weight of ‘32’, the brightness by one sustain pulse pair, the brightness by twelve reset pulses, and the brightness by one address discharge.
Herein, if the values of ‘r’, ‘a’ and ‘s’ are substituted in the gray level of ‘31’, the brightness of ‘20,61184’ is expressed in the PDP. Further, if the values of ‘r’, ‘a’ and ‘s’ are substituted in the gray level of ‘32’, the brightness of ‘17.62166’ is expressed in the PDP. That is, there occurs the gray level inversion in the related art PDP. Accordingly, it is not possible to display a picture with a linear brightness.
On the other hand, the gray level inversion is generated not only in the driving method of the sub-field, but also in a driving apparatus as in FIG. 5.
FIG. 5 is a diagram representing a driving apparatus of a plasma display panel of the related art.
Referring to FIG. 5, the driving apparatus of the related art PDP includes a first reverse gamma corrector 32A, a gain controller 34, a error diffuser 36, a sub-mapping unit 38 and a data aligner 40 connected between an input line 1 and a panel 46: and a second reverse gamma corrector 32B, an average picture level APL part 42, and a waveform generator 44 connected between the input line 1 and the panel 46.
The first and second reverse gamma correctors 32A and 32B perform reverse gamma correction on a gamma corrected video signal to linearly convert the brightness value in accordance with the gray level value of the video signal.
The APL part 42 receives the video data corrected by the second reverse gamma corrector 32B to generate Y (Y is a natural number) step signal for controlling the number of sustain pulses. The gain controller 34 amplifies the corrected video data from the first reverse gamma corrector 32A by as much as effective gain.
The error diffuser 36 diffuses an error component of a cell to adjacent cells to finely control the brightness value. The sub-field mapping unit 38 re-allots the video data corrected from the error diffuser 36 by sub-fields.
The data aligner 40 converts the video data inputted from the sub-field mapping unit 38 to be suitable for the resolution format of the panel 46, and then supplies to an address driving integrated circuit (hereinafter, referred to as IC) of the panel 46.
The waveform generator 44 generates a timing control signal by the inputted Y step signal from the APL part 42 and supplies the generated timing control signal to an address drive IC, a scan drive IC and a sustain drive IC of the panel 46.
In the driving apparatus of the related art PDP, the APL part 42 receives the video data and calculates the APL step in accordance with the inputted video data. At this moment, the number of sustain pulses is determined to correspond to the APL step. In the APL part 42, the number of sustain pulses, as shown in FIG. 6, is set to have an inverse proportional relationship with the APL step. That is, the number of sustain pulses decreases as the APL step increases, and the number of sustain pulses increases as the APL step decreases. In this way, if the APL step and the number of sustain pulses have the inverse relationship, power consumption can be sustained uniformly.
On the other hand, the APL part 42 determines the number of sustain pulses by way of rounding to the nearest whole number when the number of sustain pulse corresponding to the APL step has a fractional value. For example, if the number of sustain pulses corresponding to a hundredth step of APL is 500.5762 in FIG. 6, the APL part 42 rounds the number of sustain pulses to the nearest whole number to set the number of sustain pulses as 501.
However, if the number of sustain pulses is rounded to be set in the APL part 42, the gray level inversion become more serious.